i965/gen7+: Ensure that color buffers are resolved when necessary.

This is parallel to the logic used to resolve depth buffers.
i965/gen7+: Ensure that front/back buffers are fast-clear resolved.

We already had code in intel_downsample_for_dri2_flush() for
downsampling front and back buffers when multisampling was in use.
This patch extends that function to perform fast color clear resolves
when necessary.

To account for the additional functionality, the function is renamed
to simply intel_resolve_for_dri2_flush().
!UPSTREAM: i965/gen7+: Allocate MCS buffers for use by fast color clears.

TODO:

- write code to ensure that color resolves happen when necessary

- write code to do fast rather than slow color clears when possible

- Create a bit to keep track of whether a resolve is necessary

- Do we need to do any extra work for window system color buffers?

- In circumstances where SwapBuffers() makes the back buffer contents
  undefined (when is this?), can we save time by doing a pre-emptive
  fast color clear?  (Seems like it would save time by avoiding
  unnecessary reads when rendering causes a cache miss) Note: a lot of
  apps probably do a glClear() as the first operation after a swap, so
  to avoid slowing down those apps, we probably would only want to do
  this if we also were able to detect redundant clears (see above).
!UPSTREAM: Temporary debugging hack to print out contents of MCS buffer
!UPSTREAM: Temporary debugging hack to test RT resolves